Contactless identification systems with contactless transmission of energy and data from a data transmission/reception device to a portable data carrier via an electrical, magnetic or electromagnetic alternating field are well known. In particular what is known as radio frequency identification (RFID) offers a possibility to contactlessly read out information located on portable data carriers or to write data thereto. Starting therefrom, RFID technology opens up a large number of possible applications; for example, it opens up possibilities for permanently checking whether for example specific goods or products are present in warehouses during production sequences or whether specific goods having specific equipment features are present at specific locations.
RFID systems have a plurality of basic components and technical properties by which they are defined. Generally provided is what is known as a reading apparatus, or reader for short, which is connected to an antenna. The reading apparatus emits a corresponding interrogation signal via the antenna. This signal, which is received by a tag, serves at the same time to supply energy to the tag. The corresponding information is read out on the tag and returned to the transmission/reception apparatus, known as the reader, which picks up and evaluates the corresponding signal via the antenna. The path is in this case a bidirectional transmission/reception path in an identical frequency range or frequency band. For this purpose, different frequency bands may be cleared in the different countries for this technology.
The aforementioned tags conventionally comprise, in addition to a substrate, for example in the form of an optionally flexible film, a data carrier antenna and also an associated circuit arrangement (chip) in which is stored the corresponding information which can be read out after reception of a signal.
In RFID technology, different types of tags and, depending on the types of tags, to some extent also different reception methods (some of which are also frequency-dependent) have become known.
The corresponding transponders, referred to hereinafter also as tags for short, differ for example in terms of the transmission frequency, but also in terms of their purpose of use.
For example, dipolar tags have become known, which draw the energy irradiated by the reader, above all from the E-field or a combination of the E and the H-field, i.e. the electromagnetic field.
In addition, somewhat small looped tags have become known, which are coupled primarily by the H-field, i.e. the magnetic field.
In addition, there are also mixed forms of transponders, i.e. tags.
Just as the tags differ from one another, i.e. in terms of whether the tags are oriented primarily to the reception or the emission of E-fields, of H-fields or to the combination, the antenna designs for RFID readers also differ from one another.
Thus, the RFID antennas which are conventionally used are patch antennas. Antennas of this type conventionally have very low selectivity in their near range.
In addition, loop antennas, in particular large loop antennas, have become known, which are suitable above all for transmission and reception by means of magnetic fields.
Thus, for example, according to US 2008/0048867 A1, the use of a somewhat rectangular or circular RFID antenna has become known, which is fitted in its circumferential direction with one or more capacitors. Ultimately, the capacitors can also be fitted by way of an interruption or a plurality of interruptions in the circumferential direction of the antenna which is, for example, in principle somewhat circular in its configuration. An antenna of this type is intended to be suitable, in particular, as a UHF RFID antenna generating a magnetic coupling to tags located in the antenna region. Nevertheless, antennas of this type generate not inconsiderable electromagnetic radiation perpendicularly to the axis of the loop, as in dipoles. In addition, in antennas of this type, a reflector has to be used in order to achieve an improvement. For this reason too, the antenna produces overall a comparatively large design, partly owing to the necessary spacing between the loop antenna and the reflector. In this case, according to this prior publication, the aim is to generate loop antennas of the type in which the length of the loop portions, which are separated from one another in each case via a capacitor, can be longer than the wavelength of the excitation signal.
A further segmented loop antenna has also become known from the publication “Segmented Magnetic Antennas for Near-field UHF RFID”, Microwave Journal and Horizon House Publications, Vol. 50, No. 6 June 2007. The antenna has in principle a polygonal shape and is highly segmented. Each individual segment is formed from a metal line comprising a capacitor, which is connected in series, relative to the next segment. This publication discloses as being known, for example, an eight-polygonal antenna with six capacitors or, for example, a sixteen-polygonal antenna with fifteen capacitors at a magnitude of 1 pF and a resistance of 10Ω.
Finally, antenna designs have also become known, in which antennas are constructed on the basis of a microstrip line. This may be taken to be known, for example, from US 2007/0268143 A1. Antennas of this type possess lengths of ≧λ/2 and are used to implement an E-field coupling. Typically, the length of an antenna of this type is greater than λ/2 (based on the operating frequency) and less than λ (wherein λ is the wavelength in the dielectric).
Antennas of this type can be embodied as non-radiating antennas, for example in the form of meander line antennas. A meander line antenna of this type is arranged on the surface of a substrate (above a ground surface) the meander line antenna being fed at one end and terminated at its opposite end using a resistor which is connected to ground.
In an antenna construction of this type, E-field coupling can be used to read out, for example, labels which are provided with suitable tags and are moved directly adjacently beyond the antenna in question.
Against this background, it is the object of the present invention to provide an improved magnetically coupling near-field RFID antenna which has a design which is as small as possible and causes in this case as little power irradiation as possible in order to ensure, in this way too, high selectivity, thus ensuring in the immediate near field of a tag passed by the antenna that at all times only a single tag located directly in the antenna region can be read out.
According to the invention, the objective is achieved in accordance with the features specified in claim 1. Advantageous configurations of the invention are specified in the sub-claims.
Within the scope of the solution according to the invention, a near-field RFID antenna is proposed, in which power irradiation is reduced to a minimum. In other words, it is possible to ensure within the scope of the invention that for example less than 20%, in particular less than 15%, 10% or even less than 8%, 6%, 4% or even 2% of the power is irradiated.
The antenna according to the invention is in this case optimized for magnetic tags, i.e. for tags which are fed and addressed primarily via the magnetic field. In this case, the antenna according to the invention generates in the near field a strong H-field.
The antenna according to the invention has in this case a loop shape which, in terms of basic shaper can readily vary in regions, for example can be circular, oval, square or somewhat rectangular, n-polygonal or the like in its configuration. The key aspect is that the antenna means, which is referred to as a looped antenna, is returned from a feed point via a closed path as close as possible to the feed point and terminated there to ground via a terminating resistor.
The near-field RFID antenna according to the invention consists of a microstrip line which is fed at one end and is terminated at the other end by the line impedance. As a result, it is possible to generate a purely progressive wave. The length L of the strip conductor of the frame-shaped antenna is in this case less than λ/2 (wherein λ is the wavelength, i.e. the operating wavelength in the dielectric). Preferably, the length is less than λ/3.
The gap between the feed point and terminating resistor is in this case ideally as small as possible, regardless of how the strip line frame antenna is specifically shaped, i.e. whether it is somewhat circular, elliptical, rectangular, etc. in its configuration. In this way, it is possible to make the direction of flow at a specific point in time uniform in all cases on the entire line, thus effectively strengthening the magnetic field and reducing the electrical field.
A further advantage of the antenna according to the invention over large magnetic (segmented) antennas consists in its broadband nature with respect to adaptation. One reason for this is the non-resonant structure of the antenna construction according to the invention.